This invention relates to tilting pan filters which travel in a continuous cyclical path and which are capable of rotating about an axis perpendicular to the direction of pan travel for discharge of filter cake. This invention is used for separating a slurry into particulate matter and liquid.
The invention particularly relates to tilting pan filters which are used in the wet-process phosphoric acid industry. The filter cake which is to be discharged is composed of gypsum. In the wet-process phosphoric acid plants it is to return some of the filtrate recovered during dewatering to rinse filter cake during various portions of the filtering cycle. The balance of the filter liquid represents product acid. The tilting pan filters of the present invention and shown in the prior art described hereunder are used for the countercurrent washing recovery of phosphoric acid from gypsum.
It is known in the art to use tilting pan devices for filtering particulate matter from liquid. In these prior art devices, liquid is supplied to consecutively cover a filtering surface of each pan, with a source of vacuum being connected to an opposite side of the filtering surface, to draw liquid therethrough. During each cycle, liquid is drawn through the filtering material leaving only the filter cake behind. Additional steps, such as washing using recycled liquid, are also known in the prior art.
The prior art filter pans as described above have several drawbacks. The prior art filtering pans have sidewalls opening outwardly along both the forward and the trailing edges, which arrangement does not provide either the maximum filtering area possible, or the maximum volume for filter cake, since the dead space between pans is not minimized. This prior art type of arrangement leads to inefficiencies due to lower liquid velocity through the filter cake, and to greater liquid viscosity losses. For a predetermined plant size, reduced filtration, or alternatively a lower quality of filtration efficiency results (defined as the rate at which particulate material is removed from the liquid), in the prior art devices.
Another drawback of the prior art devices is the splashing of feed liquid and wash liquid as it flows in a wave about the pan before losing the kinetic energy imparted during feeding. The sidewalls on prior art pans do not contain the liquid to the maximum extent possible, as they generally scope outward on both the forward and trailing edges. Liquid and slurry can fall between pans, reducing filtration rate and efficiency and increasing maintenance requirements.
A tilting motion is necessary in order to empty a filter pan of the filter cake at the end of each cycle. The outwardly-opening sidewalls of the prior art are thought to facilitate discharge of the filter cake.
It is a problem with prior art pans that they do not drain the filtered liquid out of the pan body rapidly, since the axis of the pan is the drain point. A pan whose filtering surface is close to the axis will not drain quickly as there can be little slope on the surfaces on which the liquid moves to the drain point. However, the pan may be in close proximity to its neighbor, as the radius of the circle on which the pan moves as it tilts is relatively small, reducing the potential filtration area wasted to prevent interference with the neighbor. A pan whose filtering surface is further away from its axis can be made to drain more quickly but the radius of the turning circle is larger, requiring more wasted area between pans to prevent interference. It is desirable for pans to drain quickly as this increases filtration rates, reduces product contamination with wash liquids, and improves filtration efficiency.
Furthermore, cloth filters are used over a liquid-pervious support bed as the filtering medium in the prior art. Hold-down devices are employed along the edges of pans, about which the cloth is wrapped. The hold-down devices are tensioned by tensioning devices mounted to the side and end walls of the pans. The hold-down devices of the prior art are relatively bulky and occupy a volume which causes the filter cake height to be increased for a given mass of filter cake. This increased cake height causes losses in filtering efficiency due to decreased fluid velocity caused by increased liquid viscosity frictional losses. It is therefore a problem with the prior art devices that hold-down devices having a relatively large volume are employed for tensioning the filtering cloth.
It is therefore a problem in the art to efficiently use the maximum filtering surface available relative to the volume of space swept cyclically by the filtering surface of each filter pan. It is also a problem in the prior art devices to permit tilting and rotating of each filter pan about a radial axis to the path of travel of the pan, to permit emptying of filter cake from each filter pan. Typical of the prior art filter pan devices are those discussed hereunder.
The Davister U.S. Pat. No. 4,330,404 is an illustration of the typical prior art pans, their function, and their accessory equipment. It teaches use of multiple filter pans traveling through a circular arc in a horizontal plane, which are rotatable about a radial pan axis to discharge the filter cake by-product, which does not pass through the filter materials. The pan then returns to an upright position to receive more fluids to be filtered. Washing or rinsing cycles can be used to increase the efficiency and/or quality of the filtering process. Clearance is maintained between each of the tilting pans, to permit the rotation about the radial axis of each pan during discharge of filter cake.
In the patent to Steinkrause, U.S. Pat. No. 4,391,706, a filter element sealing device for a filter pan is shown in FIGS. 4 and 6. This device is of a resilient material and is generally L-shaped. It is held down by a hold-down arm which is held in place against a wall of the filter pan by a wedge.
In U.S. Pat. No. 3,830,658, to Davister, a method and device for washing a continuous filter is shown. This invention teaches cleaning of tilting filter pans wherein wash water is used to scour the filter material while the filter pan is inverted, the wash water being supplied between the bottom of the tilting filter pan and the filter material.
In U.S. Pat. No. 4,172,791, to Davister, a multiple cell filter having a gas discharge is taught. The gas discharge is shown in FIG. 8, and in FIG. 1 an entire schematic diagram of the liquid-removal system employing a vacuum is shown.
In the patent to Lyle, Jr., et al, U.S. Pat. No. 3,327,860, a tilting pan filter drain valve is shown. This valve is shown in FIG. 3, and provides a selectively openable drain in the filter pan.
In U.S. Pat. No. 3,389,800 to Roos, a tilting pan filter having a residual filtered liquid trap is shown. In another patent issued to Roos, U.S. Pat. No. 3,216,576, a filter pan structure is taught.
In U.S. Pat. No. 3,139,404, to Stock, a tilting filter pan device is shown having filter support structural details as well as a means for causing rotation of the tilting filter pans, namely rollers 50,52 in FIG. 1.
In U.S. Pat. No. 3,426,909 to Garner, a support grid for a filter pan is shown.